April 17, 2014

Martian Meteorites Offer Evidence Of Early Atmosphere On Red Planet

Martian meteorites that fell to Earth are revealing secrets of an early atmosphere on Mars that is hidden in the chemical signatures of each ancient rock. Geologists, who collected and studied 40 of these otherworldly rocks, have found an important key to how the atmospheres of Mars and Earth diverged very early in the evolution of the Solar System.

The results of this study, published in the journal Nature, will help guide researchers’ next steps in understanding whether life exists, or has ever existed, on Mars. The results may also give answers as to how water – now absent from the Martian surface – flowed there in the past.

The research team, led by Heather Franz, a former University of Maryland research associate who now works on the Curiosity team at NASA Goddard Space Flight Center, and including James Farquhar, a UMD geologist, measured the sulfur composition of 40 Martian meteorites – the largest number of Martian rocks analyzed for a single study to date. The researchers said that of the 60,000 meteorites ever discovered on Earth, less than 0.02 percent (69 meteorites) are believed to be pieces that were blasted off the surface of Mars in its early history.

The analyses of the Martian rocks showed that they were igneous and were ejected into space when an asteroid or comet slammed into the Red Planet, catapulting some of them toward Earth. The oldest meteorite in the study is about 4.1 billion years old, only 500 million years younger than the Solar System itself. The youngest rocks are between 200 million and 500 million years old.

By studying meteorites of varying ages, the team said that they can investigate the chemical composition of the Martian atmosphere at different periods in its history and learn whether the planet was ever capable of supporting life.

Both Mars and Earth share the basic elements for life, but the current conditions on the Red Planet are not favorable for sustaining life. The surface of Mars is arid and cold, is bombarded by radioactive cosmic rays and ultraviolet radiation from the Sun. But the evidence in the rocks show that some Martian geological features were formed by water, indicating the environment was not as hostile earlier in its life.

Franz and her colleagues are not sure exactly what conditions on mars made it possible for liquid water to exist on the surface, but greenhouse gases released by volcanic eruptions likely had some role.

The finding of sulfur in the Martian meteorites bolsters the fact that sulfur is plentiful on Mars, and may have been among the greenhouse gases that warmed the surface, which in turn may have provided a food source for microbes.

In the Martian meteorites, Franz and her team found that some of sulfur did in fact come from molten rock (magma), which would have come to the surface during volcanic eruptions. These volcanoes also would have vented sulfur dioxide into the atmosphere, where it interacted with light, reacted with other molecules, and then settled back on the surface.

The analyses of the rocks included measuring the sulfur isotopes. Sulfur has four naturally occurring isotopes, each with its own chemical signature; because each isotope reacts with elements in a different way, the team can determine whether the sulfur was from magma deep below the surface, atmospheric sulfur dioxide, or perhaps the product of biological activity.

The team turned to some high-tech analyses to track the sulfur isotopes in the rocks, discovering that some sulfur was a product of photochemical processes in the Martian atmosphere. The sulfur would have been deposited back on the surface and later incorporated into erupting magma that formed igneous rocks. The isotopes found in the samples are also much different than those that would have been produced by sulfur-based life forms.

Franz and colleagues also found the chemical reactions involving sulfur in the Martian atmosphere were different than those that took place early in Earth’s geological history. This suggests that the two planets’ early atmospheres were very different.

It is unclear what these exact differences are, the team noted, but other evidence suggests that soon after the Solar System formed, Mars lost most of its atmosphere, leaving it much thinner than that of Earth’s, with lower amounts of carbon dioxide and other gases. This is one reason why Mars today is too cold for liquid water to exist on the surface.

However, Franz admitted, this was not always the case.

“Climate models show that a moderate abundance of sulfur dioxide in the atmosphere after volcanic episodes, which have occurred throughout Mars’ history, could have produced a warming effect which may have allowed liquid water to exist at the surface for extended periods,” Franz said in a statement. “Our measurements of sulfur in Martian meteorites narrow the range of possible atmospheric compositions, since the pattern of isotopes that we observe points to a distinctive type of photochemical activity on Mars, different from that on early Earth.”

Franz said periods of higher levels of sulfur dioxide may help explain the dry lakebeds, river channels and other evidence of a watery past on the Martian surface. Warm conditions may have even persisted long enough for microbial life to develop.

The work of Franz, Farquhar and colleagues have yielded the most comprehensive record of the distribution of sulfur isotopes on Mars. In effect, they have compiled a database of atomic fingerprints that provide a standard of comparison for sulfur-containing samples collected by NASA’s Curiosity rover and future Mars missions.

Farquhar noted that the information from these samples will make it much easier for researchers to zero in on any signs of biologically-produced sulfur that may exist on the Red Planet.